This invention relates in general to apparatus and method for detecting data signals contained on a magnetic medium and, in particular, to an apparatus and method for detecting data signals occurring after the end signal on a magnetic medium.
At the present time credit cards are used extensively for a multitude of purposes; for example, for electronic funds transfer. It is envisioned that credit cards will be used to initiate electronic funds transfer to an even greater and greater extent in the future. One way of reading the binary information stored on the magnetic stripe of the credit card is through a manual stripe encoded card reader. The use of such a card reader requires a user to insert the card into the reader and thereafter push or pull the card through the reader to allow the entire magnetic stripe to pass by the read head.
The magnetic stripes which are read are standardized by the American Banking Association. That is, there are a number of standardized signals present on the magnetic stripe. For example, at approximately 0.293 in. from the inserted edge of the credit card a start signal typically appears. The start signal comprises information bits which are disposed between clock pulses which are equidistantly spaced along the magnetic stripe. The magnetic stripe must also include an end signal which signals the end of information bits. An LRC signal occurs immediately after the end signal, which LRC signal is a mod 2 summation of all of the data bits found on the stripe to indicate the entire credit card message.
One problem associated with reading this binary information from the magnetic stripe is that of jitter while the message is being read. Jitter most frequently occurs at the beginning and at the end of the magnetic stripe due to wear and other factors. A prior patent application entitled "A Digital Rate Compensation For A Card Reader", application Ser. No. 672,999 filed on Apr. 2, 1976 by Eugene Banka and Gerald Lehner and assigned to Burroughs Corporation, describes and claims not only a card reader speed compensation system but also apparatus and method for overcoming the jitter problems at the beginning of the magnetic stripe. The present invention describes and claims an apparatus and method for overcoming the jitter problem occurring at the end of a magnetic stripe to thereby reduce the number of undetected errors read from the magnetic stripe.
Jitter not only causes read errors such as parity misreads but even more critical, leads to undetected read errors. An undetected read error can occur if jitter upsets the data recovery logic of the module and the logic thinks that it sees an end signal with the correct parity. The odds of this happening are 1 in 32, assuming a 5 bit binary word with one of the bits being used as a parity bit. For an undetected read error to occur, the LRC signal must also be correct and the odds of this happening are also 1 in 32. Therefore, the odds that an undetected read error can occur are 1 in 1,024.
Since the American Banking Association requires that at least twelve clock bits be provided after the LRC signal on the magnetic stripe, by checking the area between a predetermined number of these clock signals for information in the form of data signals, the odds of undetected read errors occurring are decreased. For example, if eight additional information bit areas are checked, the odds of an undetected read error occurring are 1 out of 262,144 rather than 1 out of 1,024. In other words, these eight bit areas are read to determine whether any information is contained therein to thereby determine whether the end signal and/or the LRC signal were read correctly. For every bit checked after the LRC signal, the odds of an undetected read error occurring go down by a factor of 2.
The present invention provides an apparatus for reducing the number of undetected errors serially read along the length of a binary information bearing medium. The read information comprises a serial self-clocking data stream having clock and data signals including a start signal, an end signal, and a summation signal following the end signal to indicate the number of data signals in the data stream. An apparatus constructed in accordance with the instant invention comprises a first detecting means for detecting the end signal and a second detecting means responsive to the first detecting means for detecting a data signal read from a predetermined portion of the medium read after the summation signal.
Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: